The present invention relates to a process for continuous crystallization of dextrose monohydrate from a mother liquor in which nucleation and growth of the nuclai are separated form each other and in which the crystal seeds are crystallized out by cooling crystallization and the crystals are separated for the mother liquor.
In conventional process for producing dextrose monohydrate, a thinned starch having a solids content of about 30 to 35 weight percent is hydrolyzed to dextrose (saccharification) by glucoamylase in a large agitated tank known as an enzyme convertor or reactor at a temperature of about 55.degree. to 60.degree. C. for about 48 to 96 hours to convert the starch to about 95 to 96% dextrose. After saccharification, the hydrolysate is usually clarified by centrifugation or precoat filtration to remove traces insoluble fat, protein and starch, and then is decolorized, as by treatment with powdered carbon, granular carbon or ion-exchange resins. The decolorized liquor then is evaporated to about 50 to 55% solids and may be given a second carbon treatment. For production of dextrose monohydrate, the decolorized liquor is evaporated under reduced pressure to a syrup containing 70 to 78 weight percent solids and then is cooled and passed to crystallizers. The common form of crystallizer used in the crystallization of dextrose monohydrate is a horizontal cylindrical tank fitted with a slowly turning agitator, cooling jacket, and cooling coils. A substantial bed of seed crystals comprising about 20 to 25 weight percent of the previous batch is left in the crystallizer and the syrup, at about 46.degree. C., is mixed with the seed crystals giving an initial temperature of about 43.degree. C. The agitated mass is slowly cooled to about 20.degree. to 30.degree. C. over a period of days. By then, about 60 weight percent of the dextrose has crystallized as the monohydrate in a form suitable for separation and washing, which is generally performed in perforated-screen centrifuge baskets.
Until now, dextrose-monohydrate crystallization processes have been discontinuous and required between 48 to 80 hours for total crystallization. In these known processes, the bed of crystal seeds in an amount of 20 to 25% of the weight of the previous batch is left in the crystallizer in order thereby to achieve a crystallization in which the crystal phase that is formed is as homogeneous as possible and corresponds to that of the seed and has only a small percentage of needles. The grain fraction obtained by discontinuous crystallization, nevertheless, includes a large range diameters. There is, furthermore, the danger of so-called "needle crystallization" in case of supersaturation as a result of excessive cooling. Moreover, the crystalline fraction can be seperated from the mother liquor only with great difficulty and with great loss of yield.